This invention relates generally to integrated circuit processes and fabrication, and more particularly, to a system and method of adhering copper to a diffusion barrier surface.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs), and large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the surface area of the interconnect is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different resistivity characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Copper is approximately ten times better than aluminum with respect to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore, care must be taken to keep copper from migrating. The migration of copper into silicon semiconductor regions is especially harmful. The conduction characteristics of the semiconductor regions are a consideration in the design of a transistors. Typically, the fabrication process is carefully controlled to produce semiconductor regions in accordance with the design. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit material. Several materials, especially metallic ones, have been suggested for use as barriers to prevent the copper diffusion process. For example, Cho et al., in the article entitled "Copper Interconnection with Tungsten Cladding for ULSI," 1991 Symposium on VLSI Technology, pg. 39, suggests the use of tungsten as a diffusion barrier. Molybdenum and titanium nitride (TiN) have also been suggested for use as copper diffusion barriers. Gardner, et al., in an article entitled "Encapsulated Copper Interconnection Devices Using Sidewall Barriers," in 1991 VMIC Conference, pg. 99, suggests the use of sidewall structures to completely encapsulate the copper. However, the adhesion of copper to these diffusion barrier materials has been, and continues to be, an IC process problem.
Copper cannot be deposited onto substrates using the conventional processes for the deposition of aluminum when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, a chemical vapor deposition (CVD) technique has been developed in the industry.
In a typical CVD process, copper is combined with a ligand, or organic compound, to make the copper volatile. That is, copper becomes an element in a compound that is vaporized into a gas. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability the copper residue to adhere itself to the selected surface.
Wang, et al. in the article "Chemical Mechanical Polishing of Copper Metalized Multi-level Interconnection Devices," 1995 VMIC Conference, pg. 505, suggests the use of one particular copper gas compound, or precursor, for improving the adhesion of copper to a TiN barrier surface. Although certain precursors may improve the copper adhesion process, variations in the diffusion barrier surfaces to which the copper is applied, and variations in the copper precursors themselves, often result in the unsatisfactory adhesion of copper to a selected surface.
It has become standard practice in the industry to apply CVD copper immediately after the deposition of the diffusion barrier material to the IC. Typically, the processes are performed in a single chamber or an interconnected cluster chamber. It has generally been thought that the copper layer has the best chance of adhering to the diffusion barrier material when the diffusion barrier material surface is clean. Hence, the diffusion barrier surface is often kept in a vacuum, or controlled environment, and the copper is deposited on the diffusion barrier as quickly as possible. However, even when copper is immediately applied to the diffusion barrier surface, problems remain in keeping the copper properly adhered. A complete understanding of why copper does not always adhere directly to a diffusion barrier surface is lacking.
It would be advantageous to employ a method of improving the adhesion of CVD copper to a diffusion barrier material surface.
It would also be advantageous if a method were employed for preparing a diffusion barrier surface, in advance of CVD copper deposition, to improve the adhesion of copper of the diffusion barrier surface.
Further, it would be advantageous if the adhesion improving process did not degrade the electrical conductivity between the deposited copper and a conductive diffusion barrier material. It would also be advantageous if the process did not disrupt the silicon bonds and structures in adjoining IC substrates.
Accordingly, a method of applying copper to selected integrated circuit surfaces is provided. The selected copper-receiving surfaces are predominately on diffusion barrier material applied to selected regions of the IC. The method comprises the steps of: exposing each selected copper-receiving surface to a reactive oxygen species; oxidizing a thin layer of the diffusion barrier material surface in response to the oxygen exposure; and stopping the exposure of the diffusion barrier material to the oxygen before the oxide layer exceeds approximately 30 angstroms (.ANG.), whereby the relatively thin oxide layer prepares the diffusion barrier material receiving surface for adhesion to copper.
In a preferred embodiment of the invention, the method includes generating the reactive oxygen species from a predominately oxygen plasma. A preferred embodiment includes generating the reactive oxygen species from an oxygen-contained plasma, with the oxygen-contained gas being selected from the group consisting of CO, NO.sub.2, N.sub.2 O, and H.sub.2 O.
The method also provides a further step of depositing CVD copper on the oxidized diffusion barrier material surface, whereby the copper is adhered to a material which prevents the diffusion of copper into regions of the IC underlying the diffusion barrier. A preferred embodiment of the invention includes using a direct plasma source having a radio frequency (RF) power level of less than approximately 200 watts to generate the plasma, whereby the relatively low energy level of the plasma ions minimizes the disruption of silicon crystalline structures. In its preferred form, the Cu-receiving surface is exposed to reactive oxygen species at a substrate temperature of less than approximately 200.degree. C. to protect the silicon crystalline structure of the IC.
An integrated circuit is also provided comprising a first substrate layer of diffusion barrier material having a surface. The integrated circuit further comprises a layer of oxide having a thickness of less than approximately 30 angstroms and a surface, the oxide layer overlying the first substrate surface. The integrated circuit also comprises a layer of copper overlying the oxide surface, whereby the oxide layer promotes adhesion between the copper layer and the first substrate surface.
The integrated circuit further comprises a second substrate layer having a surface underlying the diffusion barrier layer, whereby the diffusion barrier prevents migration of the copper into the second substrate layer. In a preferred embodiment of the invention the diffusion barrier material is conductive and selected from the group consisting of TiN, TiON, TiSiN, TaSiN, TaN, TiW, TiWN, Mo, and WN, whereby the copper layer is adhered to a barrier material which permits electrical communication between the copper layer and the second substrate surface.
A co-pending application Serial No. 08/717,315, filed Sep. 20, 1996, entitled "Copper Adhered to a Diffusion Barrier Surface and Method for Same", invented by Lawrence Charneski and Tue Nguyen, Docket No. SMT 243, which is assigned to the same assignees as the instant patent, discloses a method for using a variety of reactive gas species to improve copper adhesion without forming an oxide layer over the diffusion barrier.
It has been standard practice in the industry to keep a diffusion barrier surface, located on a selected surface of an IC, in a controlled environment whenever possible, and to apply the copper as quickly as possible. This practice is based on the belief that protecting the diffusion barrier from uncontrolled gas environments, and keeping the barrier clean, provide the best foundation for copper adhesion. However, as demonstrated in the present invention, a thin layer of oxide promotes chemical bonding between the copper layer and the diffusion barrier surface. An oxide thickness of approximately 30 angstroms, or less, is thick enough to promote chemical bonding, and thin enough to allow the tunneling of electrons between the copper and the diffusion barrier so that electrical conductivity is not degraded.
The above disclosed preparation of the diffusion barrier surface for a deposition of CVD copper significantly improves the adhesion of deposited copper to the diffusion barrier surface. The thin layer of oxide, formed by exposure of the diffusion barrier surface to the reactive oxygen atoms, improves the chemical bonding between the copper and diffusion barrier material. Because of the relative thinness of the oxide layer, electrical conductivity between the copper and the diffusion barrier surface is not adversely affected.
The low power levels and temperatures required to perform this process insure that minimum damage is done to the associated substrates in the integrated circuit. Since the plasma exposure process is generally completed in less than 60 seconds, a minimum of damage is done to the IC crystalline structures and a speedy, commercially viable, process is insured. The improved adhesion resulting from the oxide layer permits a greater degree of variation in the uniformity of the diffusion barrier surface and the precursor. Further, the oxidation process and the copper deposition processes can be carried out in different chambers, and at different times, because of the reduced concern over the cleanliness of the processed diffusion barrier surface.